...the most significant trend shaping the region today is something different: Sunnis vs. Shiites. That sectarian struggle now infects almost every aspect of the region’s politics....Though there always was tension, Sunnis and Shiites did live in peace, for the most part, until recently.....The pivotal shift took place in 1979. The Islamic Revolution in Iran brought to power an aggressively religious ruling class, determined to export its ideas and support Shiites in the region.....Saudi Arabia is facing a series of challenges, from the Islamic State to domestic extremists. The country’s large and active social media are dominated by radical Islamists. And as oil prices plunge, government revenue has collapsed, and the nation’s generous subsidies to its people will be hard to sustain. The regime needs greater legitimacy.